In high power Nd:YAG laser,because of some pump energy transforming to heat and accumulating in laser media,and inhomogeneous cooling,which leads to the destribution of temperature in media nouniform. Furthermore the grads of temperature lead to heat effect,such as stress birefringence,thermal lens. In this paper the stress birefringence in the media is calculated when the media is under the condition of given destribution of temperature and stress;the loss of thermal depolarization is calculated when polarization light propagates though this media. The results of calculations are consistent with the experiment. Summarizing a simple method of numerical calculating stress birefringence,and developing relevant MATLAB program,which prodives reference for weakening stress birefringence and compensating the loss of thermal depolarization.